Evolution, Medicine, and Public Health [2015] pp. 195–203
doi:10.1093/emph/eov015
The evolution of capturemyopathy in hoovedmammals:a model for human stresscardiomyopathy?Daniel T. Blumstein*,1, Janet Buckner1, Sajan Shah1, Shane Patel1, Michael E. Alfaro1
and Barbara Natterson-Horowitz1,2
1Department of Ecology and Evolutionary Biology, University of California, 621 Charles E. Young Drive South, Los Angeles,
CA 90095-1606, USA; 2David Geffen School of Medicine at UCLA, Division of Cardiology, 650 Charles E. Young Drive
South, A2-237, Los Angeles, CA 90095, USA
*Corresponding author. Department of Ecology and Evolutionary Biology, University of California, 621 Charles E. Young
Drive South, Los Angeles, CA 90095-1606, USA. Tel: +1-310-267-4746; E-mail: [email protected]
Received 17 March 2015; revised version accepted 2 July 2015
A B S T R A C T
Background and objectives: Capture myopathy (CM) syndromes in wildlife may be a model for human
stress cardiomyopathy, including Takotsubo cardiomyopathy. Emotional stress or grief may trigger
heart attack-like symptoms, and occasionally, sudden death in some humans. Similarly, wildlife exposed
to predatory stresses, chase, or capture occasionally results in sudden death. To better understand the
nature of vulnerability to stress-induced sudden death, we studied cases of CM in hooved mammals—
ungulates—and hypothesized that CM would be associated with a syndrome of longevity-related traits.
Methodology: We reconstructed the evolution of CM in ungulates then determined how a set of life
history traits explained variation in the likelihood that CM was reported.
Results: CM is broadly reported, but not in all genera, and phylogenetic analyses suggest that it is an
evolutionarily labile trait. We found that the following traits were significantly associated with reports of
CM: greater brain mass, faster maximum running speed, greater minimum group size and greater
maximum longevity.
Conclusions and implications: CM may be an unavoidable consequence of adaptations to reduce pre-
dation risk that include increased running speed, sociality and having larger brains. Moreover, longer-
lived species seem to be more likely to be susceptible to CM. Exploring variable susceptibility to CM
highlights the evolutionary origins of the disorder, potential basic mechanisms that underlie vulnerabil-
ity to the phenomenon, and the potential for reduction of risk through modification of life history
trajectory.
original
research
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K E Y W O R D S : evolutionary epidemiology; capture myopathy, Takotsubo Syndrome; comparative
medicine
INTRODUCTION
Capture myopathy (CM) in wildlife may be a model
for stress cardiomyopathy (e.g. Takotsubo
Syndrome, Broken-Heart Syndrome) in humans
[1]. Peracute CM (a sub-type of CM in which symp-
toms emerge soon after exposure to the stressor) in
particular, shares many features with Takotsubo
Syndrome/Broken-Heart Syndrome in humans.
CM syndromes have been broadly described and
are varied in nature. CM syndromes are referred to
in the literature by a variety of other names including
cramp, exertional myopathy, exertional rhabdo-
myolysis, muscular dystrophy, overstraining dis-
ease, white muscle disease, muscular necrosis,
idiopathic muscular necrosis, stress myopathy and
capture paresis [2]. These syndromes typically refer-
ence the systemic consequences of damage to the
skeletal and cardiac muscles subsequent to the pur-
suit, capture, holding, handling and manipulation of
an animal. The varied nature of these systemic con-
sequences in animals including ataxia, paralysis,
rhabdomyolysis and fatal metabolic perturbations
have possibly obscured an important linkage be-
tween the more acute and probably cardiac peracute
CM and acute stress CM in humans [1, 2].
Fear and distress, independent of chase and/or
restraint are important factors in the aetiology of
all forms of CM. Typical histologic lesions including
small areas of necrosis and occasional capillary
microthrombii within skeletal muscle and other
organs are commonly reported. The diagnosis of
CM is often made when these histologic findings
are noted in association with a characteristic set of
events. It is notable that characteristic histologic
findings have been seen in free-ranging ungulates
that had not been pursued [3] and in animals killed
by hunters or dogs, and those hit by cars [4]. Sudden
/surprise attacks by predators can cause CM without
extensive chase [5]. The internal experience of fear
triggers robust autonomic responses in terrified ani-
mals. The physiological effects of these responses
contribute to all types of CM.
CM can be subdivided into three or four syn-
dromes based on the time required for symptoms
to appear, or by a mixture of time and the major
signs and symptoms that can be observed. The three
time-based syndromes are: peracute (characterized
by hyperkalemia, cardiac fibrillation and death [6, 7],
sub-acute (characterized by tubular nephrosis, renal
failure and death [8]; and chronic (characterized by
congestive heart failure, and death [8, 9]. Physical
impairment such as lameness or loss of the ability
to walk or fly can occur in non-lethal cases or before
death in lethal cases.
CM likely can occur in any animal but it is believed
to occur more frequently in mammals and birds, and
particularly in terrestrial ungulates—artiodactyls
(even toed ungulates such as camels, deer, oxen
and pigs) and perissodactyls (odd toed ungulates
such a horses, tapirs and rhinoceroses)—and in
long-legged birds such as flamingos and shorebirds
[10–14]. The reasons for such increased prevalence
are unknown.
In a series of comparative studies, we study first
the evolution of CM by reconstructing its origin in a
particularly well-studied mammalian clade—
ungulates. We report that susceptibility to CM is a
labile trait meaning that it has evolved and been lost
repeatedly over evolutionary time. Given this lability,
we then examined life history correlates of reports of
CM in ungulates. These species fall prey to a variety
of carnivores [15] and, because predatory capture
may trigger CM, we expected that they might be gen-
erally more susceptible to this syndrome [2]. For
these reasons, ungulates make an appropriate bio-
logical model to study the covariates of CM. We
focused on members of this clade to identify life
history traits associated with susceptibility to CM.
Throughout, our goal was to identify the evolu-
tionary roots of CM with the overarching goals of
developing new model systems in which to study it
as well as to identify relatively more susceptible and
relatively less susceptible species.
METHODS
Developing the comparative database
We searched for publications that alluded to,
described, and/or diagnosed capture related myop-
athy (initially in mammals and birds but later just on
the well-studied ungulates) using Google Scholar
and Scopus, using the terms ‘capture myopathy’,
‘exertional myopathy’ or ‘exertional rhabdomyoly-
sis’. We then examined the articles listed by search
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engines as citing the identified articles.
Identification and collection of articles ended in
September 2013. Articles were included in our ana-
lysis if they identified CM in the death of animals or
described the capture as the cause of death.
Instances of vital injuries to the animal in the course
of capture were not considered cases of CM; only
those that described some physiological response
to capture leading to eventual death were included.
Studying the evolution of CM
To determine the phylogenetic distribution of re-
ported CM cases we created presence/absence
matrices for the trait ‘CM susceptibility.’ Species
with reports for cases of CM were scored as suscep-
tible to the syndrome while species with no such
reports in the literature were scored as non-
susceptible. We obtained phylogenetic trees from
previous studies (mammals: [16]; ungulates [17]).
Dense sampling of characters within a phylogeny
allows for the reconstruction of trait evolution.
However, missing data can have a profound effect
on ancestral state reconstruction and estimates of
trait evolution. Because most species of mammals
and birds have not been rigorously assessed for the
presence/absence of CM, we did not attempt to es-
timate ancestral states for these groups. However,
because many ungulate species have shared a close
association with humans as either game or through
domestication, we chose to perform likelihood-
based ancestral state reconstruction of the ‘ungu-
late’ subtree of Ref. [17].
We reconstructed ancestral states in ungulates for
the trait ‘CM susceptibility’ under maximum likeli-
hood using the function Ancestral Character
Estimation in the R package Analyses of
Phylogenetics and Evolution (APE package—[18]).
Assuming that the character data and the phylogeny
is true and complete, the Ancestral Character
Estimation function allows us to estimate a history
of character evolution, evaluate the degree of uncer-
tainty associated with those estimations and com-
pare performance of different models of character
evolution. We used Akaike Information Criterion
(AIC) scores to compare the fit of the equal rates
(ER) model, which assumes that gains and losses
of CM are equal, to the all rates different (ARD)
models that allows for differences in the rate of gain
and loss of CM. An AIC difference of 4 or greater
between the best model and the alternative was
interpreted as providing substantial support for that
model [19].
Studying life history correlates of CM
For these analyses, we wished to maximize the life
history variation and therefore focused on the clade
of mammals known traditional ungulates—
artiodactyls and perissodactyls. Amongst the mam-
mals, this clade is relatively well studied with respect
both to reports of CM as well as data on a variety of
life history traits. We focused on life history traits
that had been directly reported to be associated with
CM, or indirectly reported to be associated with
stress or longevity. We included longevity because
CM causes death, so variables associated with life-
span may reflect the susceptibility to mortality-
inducing events, like CM.
Body size, which itself is a life history trait, also has
a profound influence on many life history traits [20].
Moreover, long-legged birds have been directly re-
ported to be more susceptible to CM [21], and body
mass may influence locomotor ability [22] and hence
escape ability [23]. For these reasons, we included
both body mass and body length as traits.
Allocation of resources to escape is a life history
trait [24]. The physiology of flight in humans
characterized by adrenergic stimulation, a system
also activated by stress. When predators chase prey,
prey are likely to be physiologically stressed while
fleeing [25]. Thus, we suspect there could be a rela-
tionship between greater physiological stress levels
and maximum running speed (which reflects a need
to flee). For this reason, we included maximum
running speed as one of our variables.
Predators select for other traits as well. By living in
larger groups, individuals may dilute predation risk
[26], but living in larger groups may itself be socially
and physiologically stressful [27]. However, such
stress-related effects are not uniformly found [27].
Nonetheless, group size is a trait that might be
related to both stress and longevity, and thus we
wished to understand its effects on the propensity
to be susceptible to CM.
Animals group to reduce predation risk [28], and
evidence suggests that brain size increases both in
response to increased sociality (e.g. [29]) as well as
increased predation risk (e.g. [30]). Allocation of en-
ergy to brain production is costly and brain mass is
associated with longevity [31]. While relatively large-
brained animals may live longer, brain size, never-
theless might be associated with vulnerability to CM.
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The age of weaning is a life history trait that should
form a syndrome with other traits because it is often
associated with longevity [32], and therefore may be
associated with susceptibility to CM. Additionally, at
the intraspecific level, greater anxiety in stressful
situations is associated with earlier weaning dates
in male Wistar rats [33].
West and Heard [34] noted that CM was more
prevalent both in very young, or very old, animals.
Maximum longevity is a trait that has been reported
for many species, thus, we asked whether it ex-
plained some variation in CM.
There are associations between pregnancy and
CM [34]. We hypothesize that because longer gesta-
tion periods, increased litter sizes and increased
number of litters per year are components that result
in longer and/or more pregnancies, species with
these characteristics will be more likely to have re-
ports of CM.
We gathered life history data for all species of un-
gulates listed in the Supplementary Material
describing presence or absence of CM: 225 species
were terrestrial, and two species lived in both the
water and on land. We searched for data for litter
size, litters per year and species longevity for each
of these 227 species [35]. We used the CRC
Handbook of Mammalian Body Masses to obtain data
for the minimum and maximum body masses of
each species [36]. The handbook typically contained
data for multiple individuals for each species. Thus,
we recorded only the individuals with the lowest and
highest body masses as the minimum and max-
imum body masses, respectively, for each species.
We then calculated an average of these two values to
obtain the midpoint body mass for each. We ob-
tained data for gestation period, weaning period,
sexual maturity of males, sexual maturity of females,
minimum group size, maximum group size, min-
imum body length and maximum body length from
Ultimate Ungulates, a curated website [37]. We
calculated the midpoint body lengths from average
of the minimum and maximum body lengths. We
subsequently found missing data values for these
traits from the Encyclopedia of Life and the Animal
Diversity Web [38]. We compiled data on maximum
running speed from previous studies [39, 40].
Finally, we acquired brain mass data from Perez-
Barberıa [41, 42].
We fitted type-III logistic regressions using SPSS
22.0 [43], where CM was the dependent variable and
each life history trait was an independent variable.
Rather than doing a multiple logistic regression, we
elected to fit a series of univariate models because
sample sizes varied across species for available life
history traits and we wanted the best evidence for
each potential relationship.
Because species values are not generally phylo-
genetically independent [44], we fitted a phylogenet-
ically corrected logistic regression using the R
package phylolm [45] and report the phyloglm coef-
ficients and alpha values (an estimate of the phylo-
genetic correlation).
RESULTS
The evolution of CM
Our ancestral state inferences are highly sensitive to
the method of reporting missing data. This is seen by
examining the results of the different reconstruc-
tions that varied the relative rates of loss and gain
of CM. The ARD model fit the data substantially bet-
ter than the ER model (AIC = 298.65, AIC = 51.84).
Under ARD, where the rate of gaining CM was twice
as fast as the rate of losing CM, our data show an
equal likelihood for presence or absence of the trait
at virtually all internal nodes (Fig. 1). Under this
scenario, our ability to infer the ancestral state of
medium and deep internal nodes is limited, because
the observed distribution of states could be ex-
plained by relatively recent or more ancient gains
of the state. These findings suggest that the trait is
somewhat evolutionarily labile.
Life history correlates of CM
We present summary statistics for both logistic re-
gression models and phylogenetically corrected
phyloglm models in Table 1. Phylogenetic alpha
values were often low, suggesting that these life his-
tory traits are conserved across species. Both ana-
lyses revealed that species that run faster are more
likely to be reported as having capture mypopthy.
The phyloglm results further suggest that species
with larger brains, those that live longer, and those
found in larger minimum group sizes are more likely
to be reported as being susceptible to CM. Because
we view these analyses as exploratory, we do not
report values corrected for multiple comparisons.
However, none of the analyses was significant
(P< 0.05) after correcting for false discovery rate
(the bold values were P = 0.1), and none was sig-
nificant with an even more conservative Bonferroni-
corrected P-critical value set to 0.0038.
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DISCUSSION
We begin with three fundamental limitations of our
data and the inferences drawn about Takotsubo
Cardiomyopathy. Like the results of other comparative
and phylogenetic analyses (and indeed like the results
of meta-analyses), our results and conclusions are
limited by the existing data. The prevalence of CM in
animals being captured for research and management
purposes is probably greatly underestimated for sev-
eral reasons, even in the very well-studied ungulates.
First, even when efforts are made to quantify and de-
scribe mortality of animals being captured, the
carcasses are rarely subjected to a thorough necropsy
that could result in a diagnosis [46]. Without a thor-
ough necropsy, the occurrence of CM can only be
presumed, and other causes of death are included in
the observed mortality rate. Second, an unknown and
unobserved proportion of CM deaths occur after the
captured animals are released and the carcasses may
not be recovered. Third, CM is probably a continuum of
effects, possibly affecting a majority, or even all ani-
mals that are captured, with an unknown portion of
those affected to the point of showing signs recogniz-
able as CM. Finally, impairment following capture and
processing may be attributed to other causes.
We must also note that CM is likely to be a com-
posite trait and may not be the same as Takotsubo
cardiomyopathy. CM syndromes typically affect
muscle tissue diffusely with affects noted in both
skeletal and cardiac muscle whereas Takotsubo
CM specifically refers to a stress-induced cardiac
muscle effects only. Both syndromes are triggered
by emotional or physical stress and present with
increased amounts of catecholamines and creatin-
ine kinase levels [34, 47]. CM is characterized further
by metabolic acidosis caused by a lactic acid buildup
from muscle degeneration [48]. Takotsubo is mainly
described by a bulge in the left ventricle, following
the surge of catecholamines, which causes left ven-
tricular dysfunction [49]. Despite the differences, CM
has pathophysiological similarities to Takotsubo
that permits us to hypothesize CM as a model.
We also recognize that none of the relationships
we explored were significant after corrections for
multiple comparisons. There is a debate about
whether and how to correct for multiple compari-
sons [50]. However, because some of the independ-
ent variables were correlated or provided somewhat
redundant measures of some important aspect of
life history variation, it is not clear what the appro-
priate correction for multiple comparisons should
Figure 1. The evolution of CM in ungulates. Maximum likeli-
hood reconstruction of the evolution of CM in the Bininda-
Emonds et al. [17] ungulate tree under the well-supported
ARD model of evolution (see text for details). Red squares
are species in which CM has been reported. The amount of
red in the circles illustrates the likelihood that CM was present
at an ancestral node
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be. Ultimately, given the nature of these data, and
the analyses, we feel that these results should be
interpreted in an exploratory way.
Despite these potential shortcomings, the current
results are intriguing. We focus on interpreting the
phyloglm results because of the significant phylogen-
etic signal in many of these traits. Susceptibility to CM
in the well-studied ungulates may be an unavoidable
consequence of adaptations to reduce predation risk.
Increased running speed, for example, requires an
explosive autonomic (sympathetic) response to
threat. Those species that are able to run fast are more
susceptible to CM, possibly because of the auto-
nomic response to threats. Sociality and larger brains,
also adaptations associated with reduced predation
risk are associated with more complex autonomic
systems. Indeed, more social species, including un-
gulates, may have larger brains [51]. Moreover,
longer-lived species are composed of individuals that
have successfully avoided predation. These species
seem to be more likely to be susceptible to CM.
This might be because they are more likely to have
encounters with predators throughout their lives.
Alternatively, it might be because longevity is part of
a syndrome of life history traits that predisposes spe-
cies towards having a susceptibility to CM.
Not all genera have reports of CM and this pro-
vides the opportunity to identify particularly
susceptible (or not susceptible) animal groups or
ecological scenarios/constraints that are associated
with this syndrome. By doing so we may compara-
tively examine the mechanisms that underlie vulner-
ability to fear induced sudden death across species.
Future studies are clearly warranted here.
Additionally, it is possible that the processes that
make a species susceptible to CM which we suggest
may be part of a syndrome to escape predators, may
be adaptive in high predation risk environments and
more costly in lower predation risk environments.
Future intraspecific studies are required that would
first document variation within a species in the pro-
pensity to experience CM, and then to quantify CM
susceptibility frequency in different habitats with dif-
ferent predation risk. Because latitude [52] and being
found on an island [53] is associated with predation
risk, future empirical studies may capitalize on these
sources of natural variation to evaluate the adaptive
hypothesis.
Aside from identifying potential future research
targets, these phylogenetic results suggest that
CM may be an unavoidable consequence of being
social and long-lived, at least in ungulates. To trans-
late this comparative finding into meaningful health
recommendations requires an additional assump-
tion: the factors that explain between species vari-
ation are the same as those that explain within
Table 1. Summary of bivariate analyses
N Logistic
regression
coefficient
Logistic
regression
P-value
Phyloglm
coefficient
Pyloglm
P-value
Alpha
Brain mass 97 2.425 0.024 2.259 0.035 0.427
Max running speed 39 0.043 0.026 0.041 0.030 0.012
Litter size 176 0.640 0.138 0.581 0.149 0.642
Gestation period 203 0.051 0.292 0.052 0.331 0.476
Min group size 136 0.035 0.253 0.092 0.034 0.012
Max group size 130 0.000 0.279 0.000 0.286 0.188
Mid body mass 106 0.000 0.781 0.001 0.148 0.012
Sexual maturity female 189 0.010 0.892 0.000 0.994 0.521
Sexual maturity male 179 0.059 0.481 0.139 0.132 0.652
Weaning age 159 0.005 0.915 0.003 0.942 0.608
Max longevity 158 0.006 0.751 0.049 0.025 0.012
Mid body length 191 0.001 0.200 0.001 0.214 0.012
Litters per year 113 0.142 0.794 0.144 0.792 0.437
Coefficients and P-values from logistic regression and phylogenetic logistic regression models. N is the number of species with sufficient data tocalculate a given analysis. Phyloglm coefficients and P-values are calculated from the phylogenetic GLM package phyloglm. Alpha is the phylogeneticcorrelation parameter calculated from phyloglm. Bold P-values are significant (P< 0.05) without corrections for multiple comparisons. None of theanalyses were significant (P< 0.05) after correcting for false discovery rate (the bold values were P = 0.1) or an even more conservative Bonferroni-corrected P-critical value set to 0.0038.
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species variation. There are a number of reasons why
this may not be so (e.g. the range of variation at the
intraspecific level is less than that at the interspecific
level), but interspecific comparative results often
allow the development of intraspecific testable
hypotheses. Future studies of human data sets
could help determine whether similar factors explain
human susceptibility to Takotsubo cardiomyopathy.
And, if so, future interventions might capitalize on
manipulating life history trajectories to reduce the
likelihood of experiencing Takotsubo cardiomyop-
athy during highly stressful episodes.
While highly speculative, such interventions may
capitalize on our understanding that animals
exposed to early stressful experiences have lasting
effects [54, 55]. For instance, in mammals individ-
uals who experience early direct exposure to preda-
tors, or more indirect maternal effects associated
with living in a high-risk environment, follow a very
different life history trajectory than individuals who
grow up in relatively safer environments. If such
early exposures prepare animals to escape quickly,
for example by reallocating energy towards develop-
ing fast twitch muscle fibres or having a highly active
sympathetic nervous system, then they might in-
crease the susceptibility to CM. If so, it the benefits
of reducing maternal stress and early childhood
stresses may have lasting impacts in terms of
reducing the likelihood of later being susceptible
to Takotsubo cardiomyopathy.
Clinical implications
A diagnosis of Takotsubo cardiomyopathy is made
when extreme stress, either psychological or physio-
logical, precedes an elevation of plasma catechol-
amines and an acute and unique form of left
ventricular dysfunction (heart failure). Intense emo-
tional states including fear and grief were initially
identified as precipitants to the syndrome in
humans. Recently, however, there has been
increased awareness of the potential for physio-
logical stresses such as sepsis, post-operative state,
and myocardial infarction to precipitate the syn-
drome as well [56, 57].
It is notable that this unique form of cardiomyop-
athy occurs in individuals who seem to be
experiencing the highest levels of acute stress. Of
course, the vast majority of human patients
experiencing high levels of either psychological or
physiological stress do not develop Takotsubo cardio-
myopathy. The fact that some do, however, suggests
that vulnerability to the syndrome is linked to the ten-
dency of an individual to mount a robust and intense
catecholamine response to stress.
That the vulnerability to Takotsubo cardiomyop-
athy varies among individuals is not surprising as
most medical syndromes have multiple contributing
causes including variation in the genetic background
and a variety of environmental factors. However, the
variability in susceptibility to Takotsubo cardiomyop-
athy among individuals point, at least in part, to a
diversity in how intense an individual’s autonomic
response to stress. Recognition of differential vulner-
ability to Takotsubo cardiomyopathy between individ-
ual humans leads to the connected but expanded
comparative questions: what are the differential
vulnerabilities across species and what are the mech-
anisms that underlie these differences?
Identifying the unique characteristics of animal
species with and without vulnerability to peracute
CM could create a pathway for human investigators
to study vulnerability in humans. For example,
comparing the functional integrity of sympathetic
nerve terminals or the endogenous catecholamine
levels in myocardial interstitial fluid across species
with differential vulnerability could explain why one
human being exposed to the same objective level of
stress is vulnerable while another is not. This, then,
could point to pharmacological or device driven
therapeutics for these individuals. Furthermore,
the ability to identify at risk populations could pro-
mote prevention strategies for vulnerable popula-
tions. Finally, the concept of evolutionarily framed
life history traits has little penetration in the world of
human medical thought and investigation. The
introduction of this perspective could spark novel
hypotheses pointing to population-based public
health oriented strategies for Takotsubo cardiomy-
opathy and related syndromes.
funding
D.T.B. is supported by NSF DEB-1119660; J.B. was both a
GAANN Fellow and a Fulbright Fellow while conducting this
work; M.E.A. is supported by the NSF.
Conflict of interest: None declared.
acknowledgement
We are extremely grateful to Daniel M. Mulcahy for develop-
ing the CM database as well as for extensive conversations
that initially stimulated this work
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